Size separation

Size separation, Sieving

1. Size Separation Size separation is the process of separation of different sizes of particles with the help of sieve, microscope or sedimentation technique. Particle size analysis is the determination of the size of particles present in the bulk substance or mixture.

2. Objectives/Applications of size separation To determine particle size for the production of tablets and capsules. To improve mixing of powders. To improve the solubility and stability of particles during production. To optimize feed rate, agitation, screening during production. Quality control of raw materials. Useful in production of tablets, capsules, suspension, emulsion, ophthalmic preparations, ointments, creams, etc.

3. Pharmacopoeal grading of powders (BP) 1. Coarse powder (10/44) All the particles pass through mesh 10 and not more than 40% pass through mesh 44. 2. Moderately coarse powder (20/60) All the particles pass through mesh 20 and not more than 40% pass through mesh 60. 3. Moderately fine powder (44/85) All the particles pass through mesh 44 and not more than 40% pass through mesh 85. 4. Fine powder (85) All the particles pass through mesh 85. 5. Very fine powder (120) All the particles pass through mesh 120.

4. Sieving/Sifting Sieving or sifting is a process by which different grades of powder are separated from each other. In the laboratory different grades of sieves are used to separate powders of different size during granulation & mixing process. Any materials is passed through sieves after size reduction to obtain uniform powder grades. After sieving remaining mass is known as “tailing”. It is not thrown away.

5. Sieves are made up of metal fitted with wires made up of steel, alloy or nylon. Each sieve is given a definite number which indicates the number of mesh present per inch.

6. Problems Particles aggregate due to electrostatic charges. Moisture can lead to particle aggregation. Overloading of sieve may result in errors.

7. Sedimentation It is based on the measurement of the rate of settlement of particles from a liquid in which it is dispersed. It is widely used & can determine particle size upto 0.5 µm. STOKES EQUATION

8. Andreasen Pipette is used to measure the particle size in laboratory. It consists of a 10 ml pipette fitted with a 500ml capacity sedimentation vessel by ground glass joint. A stopcock is fitted in pipette discharge the sample. A suspension ( upto 2%) of powder should be prepared in a suitable liquid. Then it is passed through sedimentation vessel at certain time intervals. The samples are dried & residues are weighed. The concentration difference can be measured with respect to time.